Among other things, modern transceivers use polar transmitters for their transmitting path. In this concept, the data to be transmitted are split into their phase and amplitude components, respectively. Amplitude and phase components are very frequently provided as digital signals. The phase component, the so-called phase modulation word, is used for driving a phase-locked loop which generates a frequency- or phase-modulated carrier signal. This is supplied to a power amplifier. The gain of the power amplifier is changed as a function of the amplitude component, the amplitude modulation word. This results in a frequency or phase modulation with simultaneous amplitude modulation of the carrier signal.
In order to ensure data transmission which is as free of errors as possible, it is necessary that the output signal of the transmitting path reproduces the input signal as linearly as possible. In the actual implementation, however, power amplifiers which have a non-linear transfer characteristic are frequently used for reasons of power consumption. Prior art FIGS. 6A and 6B show a so-called amplitude/amplitude distortion (AM/AM) or amplitude/phase distortion (AM/PM), respectively, of a typical power amplifier. Thus, FIG. 6A shows that the output amplitude AAM of a power amplifier is dependent on the input amplitude EAM of a signal into the power amplifier. Similarly the output phase PM of the amplified signal delivered by the power amplifier also changes based on the input amplitude AM of the signal as shown in FIG. 6B.
To correct these distortions called AM/AM or AM/PM distortion, respectively, predistortion units are provided in the transmit paths of modern communication devices. There, coefficients are selected based on the input amplitude, and are used to predistort the input signal so that a possibly ideal amplified signal is produced at the output of the power amplifier.
Depending on the parameters to be taken into consideration, however, the number of predistortion coefficients to be used is very large. This results in large memory consumption and elaborate calibration measures during the production.
Another possibility, therefore, consists in feeding back the signal delivered by the amplifier and determining the distortion caused by the individual elements of the transmit path. This then allows the predistortion coefficients to be determined. FIG. 5 shows such a prior art embodiment of a polar transmitter in a transmit path. In this arrangement, the carrier signal, amplitude-modulated by the power amplifier 2, is delivered at its output 22 and radiated via the antenna 5. A part of the output signal is coupled out by a directional coupler 31 and supplied to a feedback path 3a. This can be formed, for example, by the received path of a transceiver. In the feedback path, demodulation occurs and conversion into the in-phase component I′ and the quadrature component Q′ is performed. During this process, the local-oscillator signal used for the reception of signals is used as shown.